1. Field of the Invention
The present invention relates to a zoom lens, and particularly to a small-size and high-zooming ratio zoom lens having a reflective element therein.
2. Description of Prior Art
In recent years, most electronic devices, such as mobile phones, personal digital assistants (PDAs), notebook computers, have been integrated with a zoom lens. Differing from the digital zoom, the zoom lens can vary zoom ratio without deteriorating the image quality.
For example, U.S. Pub. No. 2005/0099700A1 disclosed a positive-negative-positive-positive 4-group zoom lens. When the zoom lens changes from a wide-angle end state to a telephoto end state, the first lens group and the third lens group are fixed in a predetermined position in an optical axis direction, and the second lens group moves towards the image side. The prior art disclosed an optical zooming structure, but it cannot achieve high zoom ratio.
Then, as a configuration of a zoom lens to provide compatibility between slimming down and strong durability, in a four-group configuration of first positive, second negative, third positive and fourth positive groups, a right-angle prism is placed in the first lens group for bending the optical path approximately 90.degree. at midpoint to form a refractive optical system, thereby shortening the length of the optical system in the thickness direction thereof (Refer to JP-A-8-248318, JP-A-2000-131610 and JP-A-2003-202500). In the configuration, the first lens group is fixed when zoomed and focused and thus a structure wherein the entire lens barrel can be housed in the imaging apparatus main body can be produced, making it possible to increase the durability as compared with the zoom lens made up of three groups described above.
However, the zoom lens described in JP-A-8-248318 is inappropriate for slimming down because the first lens group is large and the overall number of lenses is also large although the zoom lens has a large zoom ratio and a bright F value. The zoom lens described in JP-A-2000-131610 has a small number of lenses and a large angle of field at a wide angle end, but the first lens group is large and the right-angle prism has a low refractive index and is hard to totally reflect and thus reflecting coat becomes necessary on the reflection face of the right-angle prism, resulting in lowering of transmittance in the whole system and an increase in the cost. The zoom lens described in JP-A-2003-202500 involves various examples, which contain some examples of compact zoom lenses with a small number of lenses, but high-refractive index and high-dispersion material is used for the right-angle prism in every example and the transmittance on the short wavelength side of a visible light beam lowers. That is, in JP-A-2003-202500, a right-angle prism of a material with the refractive index at the d-line exceeding 1.8 and the Abbe number less than 30 is adopted as an example; under present circumstances, however, such a high-refractive index and high-dispersion material having sufficiently high transmittance does not exist and is not practical.